If you DON'T send any header specifying your preference regarding convenience versus privacy, you get the default behavior. That's the DEFINITION of default, what happens when it's unspecified. Think about that for second and you'll realize that's true (assuming you're not stupid, of course).
Since sending no extra header at all gets you the default behavior (by definition) , any extra header is useful only if it indicates something other than the default.
Therefore, for a browser to send extra headers specifying by default is useless, no matter what the context, by the very definition of the word "default".
You claimed a user has to be technically advanced in order to choose. The browser could have just as easily given the user a dialog with these two checkboxes:
Do you prefer web sites to provide you more convenience, or more privacy?
O More convenience and features
O More privacy
Had they done that, the header would have had meaning - it would have indicated something about the user's preference.
On the other hand, since the browser sent DNT by default, that means only that the user might prefer the default balance between convenience and privacy- exactly the same thing as not sending any header at all.